Infants are born highly sensitive to the natural patterns found in languages. They use their perceptual sensitivity to acquire detailed information about the structure of languages in their environment. To date, most studies of infant perception and early language acquisition have investigated spoken/auditory languages and hearing infants (e.g. Bosch & Sebastian-Galles, 2003; Golinkoff et al., 2013; Kuhl, 2007a; Nazzi et al., 1998; Werker et al., 1997). The few studies that have used signed language input with hearing non-sign-exposed infants indicate that infants recognize signed languages as linguistic input even without any prior experience with visual languages (e.g. Krentz & Corina, 2008; Nacar et al., 2017; Stone et al., 2017). These findings have contributed greatly to our understanding of early cognitive and linguistic development, but several questions remain unanswered. Most critically, few studies have examined deaf infants' early perceptual skills, or identified the features that infants attend to when watching signed language. The current research project aims to address these knowledge gaps by exploring how deaf and hearing infants perceive signed language input. I designed and ran a novel online preferential looking paradigm (PLP) with deaf (n=14) and hearing (n=29) infants ages 5-18 months. The infants watched videos of a bilingual deaf signer producing two unknown signed languages, Russian Sign Language (RSL) and German Sign Language (DGS). I analyzed their looking time to each language; if they watched one language more than the other, I could determine that they were able to discriminate between the two languages. I also looked at infants' attention to American Sign Language (ASL) versus an invented manual communication system (Signing Exact English, SEE) to see if infants preferred one over the other. I found that both deaf and hearing infants could discriminate between unknown signed languages and I discovered differences between the deaf and hearing infants' responses and preferences. In this dissertation, I present an analysis of the prosodic and segmental features of the stimuli as a starting point for a discussion of how signed languages vary. An exploratory analysis of the infants' responses suggests that some aspects of signed languages are salient to all infants regardless of prior experience, while attention to other features may be shaped by early experience. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]